It's a bug in the vBulletin software, which they are aware of. It's on the list of things that forum admins can vote for to be fixed, but how far down I don't know.
To repeat my off-topic reference to it yesterday in another thread, here it is from the horses's mouth (vBulletin senior tech):
"They need to make sure their orientation is correct before they upload them. We don't use the EXIF data to determine orientation. Facebook goes out of their way to fix it. We do not yet do this."
Which means, in layman's terms:
"The user will need to manually pre-rotate their images in a paint program because we're too lazy/cheap/whatever to read the EXIF data and figure it out for them".
It's up to vBulletin to fix it, so far as I can tell. Forum owners can "vote" on it being an important feature to fix here:
https://www.vbulletin.com/forum/foru...-auto-rotating
Thanks for the tip on the volume buttons, I'll give that a shot!